Recently, there has been a growing demand for coated printing papers in the field of commercial printing for advertising or publicity purposes such as advertising leaflets, catalogs, pamphlets, direct mails, etc. It would be desirable to provide these commercial prints with good print finish at low cost because it is important to achieve their purposes as advertising media though their own commercial value is low.
Especially, coated papers have been rapidly downgraded by basis weight reduction to save costs of printing papers since the recent era of low growth started. The background of the basis weight reduction of commercial printing papers is explained by cost saving as economic measures to deal with a decline in corporate profits invited by rising material prices or to reduce transportation costs, as well as social needs such as resource saving and environmental issues. In the mail-order business expanding in the past several years, the demand for basis weight reduction has also been increasing to reduce delivery costs and storage costs of catalogs and direct mails and to increase their pages.
Even in such a trend toward basis weight reduction of printing papers, it would also be desirable to maintain print quality of coated printing papers. Specifically, it is especially important to provide high opacity and resistance to print through (especially, resistance to ink penetration through paper during printing), and it is desirable to provide good brightness, sheet gloss and quality after printing, i.e., print finish.
However, basis weight reduction is incompatible to opacity and reproducibility of printed images so that it is technically difficult to achieve basis weight reduction and high print quality in printing papers at the same time, and the especially important property of opacity would be compromised by basis weight reduction.
Generally, opacity of paper mainly depends on scattering and absorption of light in paper layers, and the more light scatters in paper layers, the less light passes through the paper, which becomes opaque. Thus, a possible way to increase opacity is to increase the absorption coefficient and to increase the scattering coefficient. However, an increase in absorption coefficient results in a decrease in the brightness of paper and leads to a quality drop of coated paper, so that it is especially important to increase the scattering coefficient of paper.
A method for obtaining paper with high opacity and low density was proposed from the aspect of chemicals, which comprises adding a bulking agent consisting of a polyhydric alcohol and a fatty acid ester to hydrophobize pulp fiber surfaces, thereby preparing a low-density paper (see patent document 1). However, this method has disadvantages such as cost increase and paper strength loss because expensive chemicals must be used.